A flash memory device is a type of EEPROM (electrically erasable programmable read only memory) and is fast becoming a common device to store information. Today's flash memory devices are being used in numerous electronic devices including, but not limited to, digital cameras, MP3 players, laptop computers, personal digital assistants (PDAs), video game consoles, and the like. It is noted that numerous printers, e.g., inkjet, laser, and dedicated photograph printers are also being configured with flash memory drives to read flash memory devices. A flash memory device provides both the speed of volatile memory (RAM-random access memory) and the data retentive qualities of non-volatile memory (ROM.-read only memory). Additionally, with continued miniaturization of components and circuitry within an electronic system, flash memory devices are well suited to be incorporated into the diminutively sized systems.
Subsequent to completing fabrication of a flash memory device, the memory device requires programming voltages including, but not limited to, gate voltage. There are many methods in which the memory device can receive the various programming voltages. It is known that the programming current into the floating gate decreases as the temperature increases. Thus, as the temperature of the memory device increases, the time required to program the memory device also increases.
When a memory device is operating at a particular temperature, e.g., 10° C., given a particular gate voltage, there is an associated programming time. When a memory device is operating at a higher temperature, e.g., 70° C., given that same particular gate voltage, there is an increase in the associated programming time. Because conventional gate voltage is a constant, and not dependent upon temperature, associated programming time can be from two to ten times longer.
FIG. 1 is a graph 100 which shows the relationship between an increase in memory device programming time, e.g., programming time 110 (vertical axis with greater time toward the top), as caused by an increase in memory device temperature, e.g., temperature 120 (horizontal axis with higher temperatures to the right). This is depicted by line 150. Line 150 shows that as temperature 120 increases, the time to program a memory device increases in a non-linear manner.
This is due to, in part, reduced kinetic energy of the tunneling electrons, degraded by the severe scattering of electrons at higher temperatures. This means the electrons pick up kinetic energy as a result of the temperature increase and, instead of the electrons in the channel being pulled up into the gate, the electrons scatter into the drain region of the memory device. This unintended source to drain current flow is, to some extent, a short to the channel.
One method to overcome the inherent problem is to increase the amount of current applied to the memory device during programming. While this may be minimally effective at lower temperatures, at higher temperatures, such as those associated with programming of flash memory devices, increasing the current merely increases the amount of electron scattering. In some instances, scattering is such that current flows through a channel and into a drain region, and a programming voltage is not drawn into a gate but is therefore scattered to a drain region of a memory device.
Thus a need exists for a way to control gate voltage in a memory device. An additional need exists for a way to control gate voltage in such a way that gate voltage can be changed dynamically when a memory device is subject to change in temperature. A further need exists for a way to control gate voltage in such a way that time to program a memory device remains constant through a wide range of temperatures.